Palm switches conventionally are connected in the control circuits of machines such as hydraulic presses and the like and are closed manually by the machine operator to start the machine cycle.
There may be and frequently are switches mounted directly on the machine but more often they are carried by a "run bar" that in turn may be mounted either on the machine or on a pedestal that can be moved from one location to another as required. In any case, the palm switch is actuated manually by the machine operator who closes it to start the machine cycle by striking or pressing the mushroom-shaped head of the switch-actuator usually with the palm of his hand. In order to prevent inadvertent actuation of the switch, several pounds pressure normally are required to actuate it. Some of these switches respond to relatively light pressure and can be actuated by the fingers alone, but others are more difficult to depress and may require as much as twelve pounds of force to bottom out.
As a practical matter, palm switches are used literally in hundreds of applications. In some instances, only one palm switch is used to start the machine cycle. In other applications, dual palm switches are used; and, in these instances, it is necessary for the machine operator to close both switches in order to begin the machine cycle. For the most part, palm switches are hit quickly and then released; but sometimes it may be necessary for the operator to hold the switch closed during the entire cycle time of the machine. In practice palm switches are placed in seemingly random fashion and there is no consistent standard for their location on or near the machine. They may be placed high or low; and dual switches may be placed far apart or close together. Some switches require a lateral motion of the hand to close them; whereas other switches require a forward motion of the hand. While palm switches ideally are located at or near waist level, it sometimes is necessary for the machine operator to reach above his head or far to one side to operate the switch, or it may be necessary for the operator to reach far to one's side in order to close the switch. Many machine operators are required to wear heavy gloves to protect their hands during operation of the machine; and many of the button guards conventionally used interfere with gloved hand operation of the switch.
In many instances, guards of various kinds are mounted in association with the palm switch to prevent inadvertent operation of the switch by the operator or by a foreign object falling across the switch button. Accidental operation of the machine, of course can be dangerous to the operator as well as to the machine.
Many of the circumstances and conditions referred to above contribute significantly to operator fatigue, and this in turn sometimes results in physical injury to the operator. Moreover, it is not unusual for the operators to have to actuate the palm switch as many as five thousand times a day and the almost constant impact of the operators hand on the switch results in physical trauma and injury to the hand. The mushroom head of the switch actuator conventionally has a diameter of only two to three inches so that the force required to actuate the switch is concentrated on a relatively small area of the hand. In fact, manipulation of the switches is a leading contributor to hand and wrist trauma among machine operators; and, accordingly hand injuries are the most prevalent cause of worker compensation payments. According to a recent study, palm switch related injuries rank third in prevalence of all musculoskeletal problems. Some of the disorders that result from the continual manual pounding of improperly designed palm buttons are carpal tunnel-syndrome, tenosynovitis, and gaglionic cysts. At the end of a work shift, many operators experience tingling sensations in their hands and numbness in their fingers. Frequently, also, many operators who are required to reach for controls that are above waist height are subjected to biomechanical conditions that impose stress on the back. Electromyography (EMG) studies have demonstrated that the extensor carpi radialis (wrist extensors) and the flexor digitorum profundus (finger flextors) experience a high susceptibility to strains and sprains.